Phenanthroline-carbolong interface suppress chemical interactions with active layer enabling long-time stable organic solar cells

To restrain the chemical reaction at cathode interface of organic solar cells, two cathode interfacial materials are synthesized by connecting phenanthroline with carbolong unit. Consequently, the D18:L8-BO based organic solar cell with double-phenanthroline-carbolong achieve the highest efficiency of 18.2%. Double-phenanthroline-carbolong with larger steric hindrance and stronger electron-withdrawing property confirms to suppress the interfacial reaction with norfullerene acceptor, resulting the most stable device. Double-phenanthroline-carbolong based device can sustain 80% of its initial efficiency for 2170 h in dark N2 atmosphere, 96 h under 85 oC and keep 68% initial efficiency after been illuminated for 2200 h, which are significantly better than bathocuproin based devices. Moreover, superb interfacial stability of double-phenanthroline-carbolong cathode interface enables thermal posttreatment of organic sub-cell in perovskite/organic tandem solar cells and obtained a remarkable efficiency of 21.7% with excellent thermal stability, which indicates the potentially wide application of phenanthroline-carbolong materials for stable and efficient solar device fabrications.

For the performance of highly efficient and stable organic solar cells cathode interfacial layers play an important role. They transfer the negative charges and block the positive charges at the cathode. In the present manuscript, the authors report a on a variety of cathode interface materials that are available for both efficient single junction organic solar cell and tandem solar cell. According to the rational molecular design strategy of carbolong substituents on the Phen, the photovoltaic performance of resulted DPC and SPC improved significantly, such as higher electrical conductivity, better matching energy levels or suitable work function modification of metal cathode. As a result, the single junction organic solar cell obtained an OPV efficiency of 18.2% which is higher than that of BCP control device.
The authors found that DPC with two carbolong substitutions can suppress the negative reaction which commonly happens between the cathode interfacial layer and non-fullerene acceptor (NFA) material, leading to enhanced device stability. In particular, the DPC based device maintained 80% of its initial PCE at elevated temperatures (85°C) for 96 h or at room temperature after 2170 h in inert N_2 atmosphere. As the reported DPC material exhibits overall better photovoltaic performance and stability, DPC offers a wide range of potential of applications in OPV community.
Just from these novel achievements, I do recommend the manuscript for publication in Nature Communications. However, there are several aspects that have to be changed/clarified/commented prior to final acceptance.
1.There are too many experimental findings listed in the introduction part. Authors should just name the 3 key findings.
2.Materials characterization section: the achieved data should be summarized in one common table (which also allows better comparison of the materials' properties).
3. Fig. 2d: kinetic energy scale in UPS does not give any information if now experimental details are declared. Actually, all relevant data are shown in Fig. S11 (with all required information on determiantion of the respective values (slopes and crossing with energy axis) 4.HOMO and LUMO orbitals are important for material analysis. The authors may add the simulated frontier molecular orbitals to the supplementary information. The energy level calculation method should be described in the experimental section of the manuscript. 5.Degradation studies (Figure 3). It is not fully clear how relevant the XPS study is. Is it required to show the materials' degrdation in the main text. Major conclusion is that BCP shows significantly different composition (according to N1s binding energies), but the conclusions is more speculative.
6.Page 19, line 340: is the variation in the 3 digit really trustworthy ? 7.Results reported on page 22 just report on the device parameters -authors should focus on the most prominent ones required for their discussion/conclusions. 8.Can the authors give any idea why DPC and SPC are advantageous to suppress interfacial reactions when used as ICL ? 9.Authors should use the identical colour code for BCP, SPC and DPC in all figures.
10.Some abbreviations must be explained at their first appearance (NB: no abbreviations in the abstract).
11.The manuscript may require some rewriting/rephrasing and inspection from a native english speaking person. The use of the article "the" sometimes seems arbitrary. The word "meanwhile" is not used in the proper sense. I also found few incomplete sentences and numerous typos). See e.g. Fig. S20 ("Bing Energy") 12.Line 385: Supplementary " Table 3." should be corrected to " Table 4".
Reviewer #3 (Remarks to the Author): In this work, the authors developed two newly synthesized cathode interfacial materials, DPC and SPC, which consisting of phenanthroline core and carbolong unit. The authors investigated their improved photovoltaic performance and utilized them in organic solar cells as well as perovskite-organic tandem solar cells. They used various methods to evaluate and compare the effect of cabolong CIMs on device efficiency and stability and confirmed that the resulting DPC can minimize the chemical reaction which indeed plays an important role in the long-term stability of corresponding photovoltaic devices. Overall, this work provides lots of valuable information for molecular design. Publication of this work is recommended in Nature Communications after minor revisions. Fig.4b, the authors investigate the charge collection efficiency (ηc) of device based on various cathode interfacial layers. However, the obtained values of DPC, SPC, and BCP are too close to distinctly compare the charge transfer ability at this layer. The authors should provide more evidence on this point.

1) In
2) What is the essential reason for the strong electron-withdrawing properties of carbolong frameworks?
3) Organometallic compounds generally suffer from poor stability. However, why the carbolong complexes are quite stable?
4) The addition reaction of alkynes and metal carbynes is interesting. It is better to add some discussion about this reaction. 5) Acronyms for nouns should appear where they are first mentioned in the manuscript, including but not limited to BCP, CIM and TPC.
6) The energy levels of commonly used materials in OPV field are summarized in supplementary figure 12, references are needed here.

Reviewer #1 (Remarks to the Author):
Lai et al. reports the synthesis of two new cathode interfacial materials (CIMs) and their applications in nonfullerene organic solar cells. The new CIMs were obtained by connecting Phen core with one or two carbolong complex. The double-phenanthrolinecarbolong (DPC) delivered high performance in organic solar cells, and also showed superior chemical stability with the active layer than the traditional BCP under heat and light. The chemical stability is important for the CIM in efficient nonfullerene solar cells. It has also been a challenging task to develop a stable one and in the meantime delivering high photovoltaic performance. The CIM reported in the work will generate high impact for solar cells application and would give a hint for future development of CIM. Thus, I would recommend its publication in Nat Commun. The following comments should be addressed: We thank reviewer for taking time to review our manuscript and giving insightful and constructive comments for the improvement of the manuscript.  Table R1 (Supplementary   Table 3) in the revised manuscript with same device structure that we applied in our paper. We also added the following discussion in the revised manuscript: "Besides, we also compared the efficiency of DPC based device with commonly used

As suggested by the reviewer, fabrication of inverted single junction cells with DPC
CIM is possible if proper deposition process is followed. We have fabricated devices with a structure of ITO/DPC/D18:L8-BO/MoO3/Ag, with best performing device presented in Fig. 5c labeled as rear cell and summarized data in Supplementary Table   5. The solvent for active layer deposition is chloroform and the concentration of DPC in MeOH varies from 0 to 5 mg/mL to obtain the optimized thickness. During the deposition process of the active layer, the contact time between DPC and solution of active layer was critical and was reduced by dynamic spin coating method to prevent potential erosion of DPC underlayer (that is, 18 μL of the active layer solution rapidly dripped onto the substrate while the CIL coated substrate was rotating at high speed).
The detailed results were summarized in Table R2 and Supplementary We also added "the DPC device with optimized concentration of 1 mg mL -1 delivered a VOC of 0.85 V, JSC of 22.86 mA cm -2 with the optimal PCE of 13.7%." in the revised manuscript. 3. How about the conductivity of DPC? Is there a self-doping effect in the DPC?
Response: We have added following discussion in revised paper according to the reviewer's concerns.
The self-doping effect of CIMs were investigated by the electron spin resonance (ESR) spectroscopy that can been seen from Fig To further resealing the self-doping effect happed between acceptor and CIM, we also tested the ESR spectra of L8-BO:CIMs. The sample was prepared by dissolving L8-BO and CIM with molar ratio of 1:1 in chloroform followed by drying in a vacuum chamber.  4. Please also provide illumination data of the tandem devices.
Response: We have added the illumination stability of BCP and DPC CIL based TSC devices, which were tested under a LED light with intensity of 1 sun in glovebox as shown in Figure R4a.

18.2%
While in perovskite/OSC tandem solar cells, with device structure presented in Fig. R5c (Fig. 5a in maintext), the front DPC layer does have absorption overlap with the organic layer. However, as is shown in Fig. R5d (Fig. 5d in maintext), light with energy above the bandgap of CsPbI2Br (~1.92 eV, i.e., light above 650 nm) was adsorbed by the perovskite layer, leaving light shorter than 650 nm being utilized by the organic D18:L8-BO layer, which DPC barely absorb according to transmittance spectra in Fig.   R5e. Therefore, the light utilization in tandem device was precisely controlled, there is no light absorption competition between DPC and organic absorber. Therefore, no matter in the conventional single junction organic device or perovskite/OSC tandem device, DPC has negligible effect on the light absorption of photovoltaic layer.
We have added the transmittance data ( Supplementary Fig. 25 in revised maintext) and corresponding description is shown as below.
"For perovskite/OSC TSC, the ICL beneath active layer of rear cell should have a high optical transmittance to ensure that more transmitted light can be utilized by rear-cell.
Therefore, we measured the transmittance of BCP and DPC film, with data depicted in Supplementary Fig. 25. Although BCP film has very high transmittance between 300 to 800 nm, the DPC film obtained with optimized concentration (1 mg/mL) shows a very high transmittance of more than 97% in the long-wavelength region from 400 nm to 1000 nm, which is good enough for the solar cell utilization. Consequently, the high 6. Please comment any specific reason of using the carbolong unit for designing the DCP.
Response: There are two mainly reasons when we use carbolong units for DPC molecular design.
1) According to previous reports, the metal-carbon triple bonds and carbon-carbon triple based dπ-pπ conjugated system can work well as electron transport materials for both organic solar cells and perovskite solar cells (Nat. Commun. 2020, 11, 4651).
Systematic studies have found that when π-conjugation skeleton was introduced to a conjugated side chain can strengthen intermolecular charge transfer and make the electron transport orderly in device (Adv. Mater. 2021, 33, 2101279). Furthermore, the dipole structure of such materials can reduce the work function of metal electrode and finally improved the device efficiency and stability (J. Am. Chem. Soc. 2021, 143, 7759 −7768). Therefore, introducing carbolong into the conjugate skeleton is a very feasible strategy to design highly efficient electron transport materials.
2) Electron transport materials containing amines or N atoms have been proved that it's very good for intramolecular charge transport and promoting device efficiency (Adv. Therefore, we chose the Phen core of BCP material and connected it with carbolong unit. According to the large steric hindrance and strong electron absorption ability of carbolong, the as created material was designed to be highly efficient with reduced reactivity of N atoms and non-fullerene acceptor materials.

Reviewer #2 (Remarks to the Author):
For the performance of highly efficient and stable organic solar cells cathode interfacial layers play an important role. They transfer the negative charges and block the positive charges at the cathode. In the present manuscript, the authors report a on a variety of cathode interface materials that are available for both efficient single junction organic solar cell and tandem solar cell. According to the rational molecular design strategy of carbolong substituents on the Phen, the photovoltaic performance of resulted DPC and SPC improved significantly, such as higher electrical conductivity, better matching energy levels or suitable work function modification of metal cathode. As a result, the single junction organic solar cell obtained an OPV efficiency of 18.2% which is higher than that of BCP control device.
The authors found that DPC with two carbolong substitutions can suppress the negative reaction which commonly happens between the cathode interfacial layer and nonfullerene acceptor (NFA) material, leading to enhanced device stability. In particular, the DPC based device maintained 80% of its initial PCE at elevated temperatures (85°C) for 96 h or at room temperature after 2170 h in inert N_2 atmosphere. As the reported DPC material exhibits overall better photovoltaic performance and stability, DPC offers a wide range of potential of applications in OPV community.
Just from these novel achievements, I do recommend the manuscript for publication in Nature Communications. However, there are several aspects that have to be changed/clarified/commented prior to final acceptance.
We thank reviewer for the insightful and valuable comments and suggestions.
1.There are too many experimental findings listed in the introduction part. Authors should just name the 3 key findings.
Response: We thanks the reviewer for the suggestion. We reduced the description of research progress in the introduction, but kept the most important reports of amine group containing CIMs which are highly related to our research work. Response: We have added the auxiliary lines necessary to obtain the data in Fig. 1d, and the specific values are presented in the maintext. Because the high conductivity of Ag electrode, the Fermi level is set to zero in binding energy axis, therefore the work functions of Ag and Ag/CIL can be determined simply by subtracting the energy difference between Fermi energy and cut-off energy with 21.2 eV (He I). We take the subtracted value and re-drew the UPS spectra as shown in Fig. R6 and Fig. 1d in revised paper. Response: We have added following discussion in revised paper according to the reviewer's suggestion.
"The simulated distribution of HOMO and LUMO orbitals was provided in Fig. R7 ( Supplementary Fig. 12 in the revised paper). There is strong π-delocalization between the Phen core and carbolong frameworks in SPC and DPC." Response: To better explain the relevance of XPS measurement, we would like to give more detailed explanation based on the content of the revised manuscript.
1) To deeply understand the different chemical reactivity between BCP, SPC and DPC with L8-BO, we have carried out electrostatic potential (ESP) to study the nucleophilic ability of those three CIMs, which is shown in Fig. 2d (in the maintext). When compared with BCP, "the extremely small ESP value around the N atom of DPC indicates that the DPC has the weakest nucleophilic ability and thus greatly reduces the chemical reactivity between the acceptor at the interface." 2) To revel the materials' degradation process, we measured 1H NMR spectra of pure L8-BO, BCP and DPC solution as well as the mix solution of L8-BO:BCP and L8-BO:DPC under 80 o C ( Fig. 2e and 2f in maintext). We found that "after the L8-BO:BCP mixed solution was heated for 2 h, the characteristic H signal in the C=C peak split to two peaks indicating that the chemical structure of L8-BO began to destroy during the heating process." However, it was confirmed that DPC could greatly suppress such chemical reaction when the Phen core connects with two carbolong units in the structure ( Fig. 2f in maintext).
3) The final chemical product of BCP and L8-BO was charactered as shown in Supplementary Fig. 15 and Fig. 16 in maintext. The degradation of L8-BO when it was attracted by BCP was described in maintext as "it was highly possible that the nitrogen atoms on the BCP react rapidly with the C=C double bond of L8-BO and then destroy its structure." Therefore, we have clearly explored the chemical reaction process that happened between BCP and L8-BO, which is the nucleophilic attack of the N atom in BCP to NFA materials. In the illumination stability part, we conducted XPS measurement by checking the chemical status of nitrogen atom in BCP or DPC to study the illumination stability of devices with different CILs. Considering the reviewer's comments and making this experiment more clearly, we added samples of L8-BO/BCP and L8-BO/DPC film without any photoaging for comparison, the results are depicted in Fig.   R8, and Fig. 3b, 3c in the revised manuscript. The related discussion has been expanded/replaced as following: "To deeply insight into the completely different effect of BCP and DPC on illumination stability of device, we carried out X-ray photoelectron spectroscopy (XPS) measurements to trace the signal of N 1s orbital. In this measurement, we firstly tested the N 1s signal of pure BCP or DPC CIL film without any photoaging as shown in Fig.   R8a and 8b (supplementary Fig. 21 in the revised paper). The characteristic N 1s peak of fresh BCP is located at 398.5 eV while DPC shifted to 399.5 eV due to the two carbolong units with strong electron withdrawing property on the structure. For the fresh BCP or DPC film that spin-coated upon L8-BO film as shown in Fig. R8c and d ( Fig. 3b and c in revised manuscript), the XPS peak of N 1s are situated at 398.5 and 399.5 eV, respectively, which are same locations with the pure BCP and DPC film. After been illuminated for 2000 h, the N 1s peak of DPC that covering upon L8-BO layer also show a stronger peak at 399.5 eV which is same location as pure DPC film.
However, the peak of L8-BO/BCP film moved to 398.9 indicating that the chemical structural of BCP changed during the photon-oxidation process." 6.Page 19, line 340: is the variation in the 3 digit really trustworthy?
Response: From the photovoltaic parameters that summarized in Table 1 of maintext, the devices based on BCP, SPC and DPC CIL can achieve relatively high device efficiency (more than 17%), which means the carrier separation efficiency inside those three devices are good, suggesting very similar level of bimolecular recombination. To answer this question, we further measured transient photocurrent (TPC) and added the following discussion in revised manuscript.
In order to further explore the exciton separation in devices with BCP, SPC or DPC CIM, we conducted transient photocurrent (TPC) measurement as shown in Fig. R9 ( Supplementary Fig. 23   Response: We thank the reviewer for this insightful comment. As we have previously discussed about the inherent robustness and restricted chemical reaction of DPC layer, DPC was sandwiched between Ag and organic BHJ layer in tandem device structure, the high thermal stability of DPC enables thermal treatment of organic above layer to achieve better crystalline and improved photovoltaic performance.
We have revised this part and made it more consistent and logical with previous discussions.
The photovoltaic parameters of individual single-junction sub-cells are summarized in Supplementary 10.Some abbreviations must be explained at their first appearance (NB: no abbreviations in the abstract).
Response: We have carefully checked the abbreviations in the revised manuscript.
11.The manuscript may require some rewriting/rephrasing and inspection from a native english speaking person. The use of the article "the" sometimes seems arbitrary. The word "meanwhile" is not used in the proper sense. I also found few incomplete sentences and numerous typos). See e.g. Fig. S20 ("Bing Energy") Response: According to the reviewer's suggestion, we have carefully check the typos and grammar issues in the revised manuscript.
Response: We have carefully checked and corrected these mistakes in the revised manuscript.

Reviewer #3 (Remarks to the Author):
In this work, the authors developed two newly synthesized cathode interfacial materials, DPC and SPC, which consisting of phenanthroline core and carbolong unit. The authors investigated their improved photovoltaic performance and utilized them in organic solar cells as well as perovskite-organic tandem solar cells. They used various methods to evaluate and compare the effect of cabolong CIMs on device efficiency and stability and confirmed that the resulting DPC can minimize the chemical reaction which indeed plays an important role in the long-term stability of corresponding photovoltaic devices.
Overall, this work provides lots of valuable information for molecular design.
Publication of this work is recommended in Nature Communications after minor revisions.
Thank the reviewer for this positive comments.  4) The addition reaction of alkynes and metal carbynes is interesting. It is better to add some discussion about this reaction.
Response: Thank the reviewer for the suggestions. We have added following discussions on this addition reaction to the revised manuscript.
"It is is the electrophilic addition of carbyne by the alkyne under the synergistic effect of H + ." 5) Acronyms for nouns should appear where they are first mentioned in the manuscript, including but not limited to BCP, CIM and TPC.
Response: We have carefully checked the acronyms and corrected in the revised manuscript.
6) The energy levels of commonly used materials in OPV field are summarized in supplementary figure 12, references are needed here.
Response: We thank the reviewer for this comment. We have summarized references in